The general construction of a multifocal ophthalmic lens is known in the art. U.S. Pat. No. 5,225,858, which is incorporated herein by reference, discloses a multifocal ophthalmic lens including a central zone circumscribed by multiple concentric, annular zones. This patent discloses a means of providing improved image quality and light intensity for near images. The improved image quality is accomplished by maintaining the near vision correction power of appropriate zones of the lens substantially constant for a major segment of the near vision correction power region of each zone, and by providing a central zone having an increased depth of focus.
The major segment of each near vision correction power region, which has a substantially constant near vision correction power, inherently reduces the depth of focus associated with far vision. The location of near focus is typically immaterial for near vision, because of the ability of the user to easily adjust the working distance of the target object. The patent discloses progressive vision correction powers in the central zone for extending the depth of focus. The increased depth of focus provided by the central zone helps to compensate for the reduction in depth of focus associated with the near vision correction power regions. This feature is particularly applicable to an intraocular lens, since the patient has minimal residual accommodation, i.e., the ability of a normal eye to see objects at different distances.
FIG. 1 shows how the multifocal ophthalmic lens 6 of the prior art focuses parallel incoming light onto the retina 10 of the eye. For the normal lighting condition with a 3 mm pupil diameter, the rays 7 pass through a far focus region of the multifocal ophthalmic lens 6, and are focused onto the retina 10. The rays 8 pass through a near region of the multifocal ophthalmic lens 6, and are focused into a region between the retina 10 and the multifocal ophthalmic lens 6.
The multifocal ophthalmic lens 6 shown in FIG. 1 shows the passage of parallel rays through the multifocal ophthalmic lens 6 in a well-lit environment. In low lighting conditions, the pupil enlarges, and additional annular zones of the multifocal ophthalmic lens 6 become operative to pass light therethrough. These additional annular regions operate to provide additional far (rays 9 in FIG. 1) and near-focus corrective powers to the multifocal ophthalmic lens 6. Presence of the additional intermediate and near rays shift the best image quality for far vision to the location in front of the retina. As a result, in low lighting conditions the best quality image of the multifocal ophthalmic lens 6 appears in a region slightly in front of the retina 7. A user looking through the multifocal ophthalmic lens 6 while driving at night, for example, may notice an undesirable halo affect around a bright source of light. The shift in the best quality image just in front of the retina 7 instead of on the retina 7 increases the halo effect making driving for some people difficult.
A problem has thus existed in the prior art of providing a multifocal ophthalmic lens, which can provide a desirable far vision correction in low lighting conditions, but which does not unnecessarily elevate halos and contrast reductions under increased pupil size which usually occurs in low lighting conditions. Thus, the prior art has been unable to produce a multifocal ophthalmic lens, which achieves a best quality image on the retina (instead of slightly in front of the retina) in low lighting conditions.
Under low lighting conditions, the best quality image of prior art multifocal ophthalmic lenses is not focused on the retina of the eye. Instead, these prior art multifocal ophthalmic lenses have a best quality image in front of the retina in low lighting conditions, which corresponds to a mean power of the multifocal ophthalmic lens being slightly higher than the far vision correction power required for the patient.